Linear Accelerators (also called “LINACS”) are widely used for a variety of tasks in a broad range of applications, including industrial applications such as Non-Destructive Testing (NDT), Security Inspection (SI), Radiotherapy (RT), electron beam processing—sterilization, and polymer curing, for example. Both accelerated electron beams, and Bremsstrahlung X-ray beams (collectively called “radiation beams” (RB)) generated by such electron beams striking a conversion target at the end of an accelerating channel, are used for various tasks. The type of RB selected is typically determined by the specific application and its requirements. In many applications, the requirements include energy variation and dose rate variation of the RB, including broad RB energy variation, for example, from 0.5 MeV to a maximum energy, which typically does not exceed 10 MeV due to neutron production and activation problems. However, in some known cases, it can reach as high as 12 MeV, 15 MeV, 20 MeV, or even higher energies. Those familiar with the art are well aware that a LINAC is a sophisticated tool that does not always run efficiently, or does not perform at all over such a broad RB operating energy range.
A LINAC includes a plurality of cavities, which gradually increase in length in the direction of the electron beam propagation to keep the particles in the right accelerating phase while their velocity increases. Once electron velocity reaches nearly the speed of light, the period of the structure and the shape of the accelerating cells usually remain the same until the end of the accelerator.
The front irregular section of the LINAC where electron velocities change substantially (from about 20% to 95% of the speed of light), and where the electrons are grouped together as a stream of bunches of electrons, is typically called the “buncher”. The buncher is responsible for forming the relativistic electron beam, which then enters the regular periodic part of the LINAC structure, called the “accelerator”, where the velocity of the electrons does not change substantially, while they reach higher energies above 1 MeV, and up to the N×10 MeV range or higher (where N is an integer 1, 2, . . . N).
An important parameter used for defining efficiency of the buncher is called “capture”, which presents a percentage of the particles captured by the accelerating fields, and synchronously accelerated to the required energy with respect to a total number of particles injected into the structure. Capture is very sensitive to the accelerating field distribution in the buncher. While one attempts regulating output energy of the produced RB by varying input RF power into the LINAC, the structure of the fields in the buncher change, and the electron beam current in the accelerating channel may reduce substantially due to degradation of capture in the buncher, thereby reducing intensity of the produced RB.
The same may be true for regulating the RB energy via switching of the injected electron beam pulse current without optimizing power and field distribution along the linear accelerator. The optimization is especially important for the magnetron-driven LINACS, which represent most of the commercial markets, and even more so, for the higher frequency LINACS (designed to operate with an X-band power source, for example) where lack of the input RF power generated by the best commercially available X-band magnetrons for a given task exists in most, if not all cases (so-called “power hungry” mode of operation).
A traditional SW LINAC has been described in multiple references, and is presented schematically in FIG. 1. RF power is provided by the RF power source (1) (which in most cases, is a magnetron or a klystron). The RF power propagates through an RF Transmitting Waveguide (2) and a High Power Circulator (3) to the Input RF Coupler (4), which is configured to match impedance of the external and internal RF circuit so as to minimize power reflections at the operating RF frequency. High Power Circulator (3) serves to prevent reflected power from propagating back to the RF source (1). It is called “High Power” rather than “Low Power” (see below) because it is adapted for the maximum possible power generated by the RF source. Therefore, most of the RF power from the RF source (1) enters the LINAC, which usually includes a plurality of single RF cavities coupled together in various ways depending on the RF structure design. FIG. 1 shows a LINAC that has two single RF structures coupled together.
The LINAC of FIG. 1 can be divided into two parts—a Standing Wave (SW) Buncher (5) and a Standing Wave (SW) Accelerator Section (23). The SW buncher (5) contains a sequence of cavities, which are different in length so as to maintain proper phase shift between the accelerating fields in the neighboring cells accommodating the gradually increasing electron velocity, which rapidly increases to relativistic values (close to the speed of light). The electron velocity becomes nearly constant in the Accelerating Section (23), so all the cells are the same in length.
The single RF cavity of the Input RF Coupler (4) is also part of the LINAC RF structure. In the case of the SW LINAC, this Input RF coupler (4) can be positioned virtually anywhere along the LINAC, but usually, it is placed somewhere after the SW Buncher (5) and before the SW Accelerator Section (23). In the LINAC of FIG. 1, the SW Buncher (5), the Input RF Coupler (4), and the SW Accelerator Section (23) together provide the single RF-coupled accelerating structure of the LINAC. The RF power is distributed among the LINAC cavities in accordance with the LINAC configuration and its RF properties, forming an RF field distribution responsible for accelerating the charged particles, for example, the electrons. Further, we will use “electrons” as the charged particles, and provide an “electron beam”, as this LINAC configuration is mostly applicable to accelerating electrons.
An Electron Beam (10) is formed in an Electron Gun (11), which can operate in a range of high voltages N×(1, 2, 3 . . . 100) kV, forming an electron beam (10) of necessary small diameter so as to enter the LINAC RF structure. The Electron Beam (10) gains energy while propagating through the RF fields of the LINAC cavities (5) and (23), and after it exits the RF accelerating structure, the Electron Beam (10) is extracted outside the vacuum envelope through a vacuum-tight thin foil for electron beam applications, or it strikes a heavy metal target to generate bremsstrahlung (X-rays) if this is the requirement for the output RB (12).
In some cases, an optional external Magnetic System (13) (such as a focusing solenoid or a permanent periodic magnet (PPM) system) is used, which may also include steering coils, bending magnets, etc. for correction of beam positioning inside the LINAC, or at its exit via Electron Beam Window or Conversion Target (12). Use of an external focusing system is undesirable because it increases complexity, power consumption, and consequently increases the cost of the LINAC system. In SW LINAC systems, use of a Magnetic System (13) can be avoided, but in TW LINACS, it is necessary in most cases, especially for the Buncher portion of a LINAC. We are not showing a TW LINAC diagram since the effects of power regulation are quite similar, and in the case of broad energy regulation, these effects are devastating to electron beam quality, just as in a SW LINAC.
To regulate energy in such a single RF feed from RF Source (1) and single SW Accelerator section (23) LINAC, field amplitude in the LINAC RF structure must be changed due to beam loading, or due to input power regulation. Analysis of the LINAC performance (in the absence of an external magnetic focusing field produced by a solenoid or a periodic permanent magnet (PPM) focusing system) is shown in the graph of FIG. 2, where a comparison is provided of a theoretical LINAC load line in the first approximation (Energy, MeV) to a load line obtained as a result of computer simulations of beam dynamic for the same SW LINAC (using Parmela computer code). The graph of FIG. 2 also presents the corresponding dose rate curve based on the first linear load line (Dose Rate, R/min@1 m) and the other dose rate dependency that corresponds to the load line based on Parmela calculations (Parmela/Dose). The effect of beam dynamics on output radiation beam characteristics is evident. A reduced complexity and reduced cost LINAC is always preferred, and a SW LINAC can be designed to avoid use of the external focusing much easier than a TW LINAC. The TW LINAC, however, delivers some properties superior to a SW LINAC, but it usually requires a focusing solenoid. A TW guide principal behavior will be similar to that for the SW, described above.
Due to a common deficit of RF power, the LINACS are usually designed for near maximum optimal output energy, where the dose rate is at its maximum defined by a well-known empirical ratio as follows:P=70×I×Wn,  (1)where:P is the Bremsstrahlung dose rate at 1 meter from a heavy metal conversion target, in R/min;I is the average electron beam current striking the target, in mA;W is the electron beam energy, in MeV; andn is a parameter that varies with energy (in several MeV range it is approximately 2.7).
For LINACS using an electron beam in a broad energy range, it is important to increase capture and efficiency at lower energy, thereby increasing accelerated beam current and electron beam dose rate of the RB. In the cases where LINACS are equipped with a conversion target so as to produce bremsstrahlung, the conversion dose rate is proportional to current, and nearly to a cube of energy. Consequently, lower energy operation of the LINAC at higher beam current becomes even more important. As we have already stated, efficient operation at lower energy is difficult to achieve, if a LINAC is designed for providing a beam at maximum energy at a given beam current to obtain the best RB output.
In the literature, one can find a variety of ways proposed by authors skilled in the art to configure LINACS such that they operate better when energy is regulated, both in a “fast” and “slow” mode of changing the beam energy.
One of the first hybrid LINACS has been proposed in patent SU 1374454 A1, where a disadvantage remains in that the SW buncher is a section of disk loaded waveguide, and it is not as efficient as other structures. In addition, regulation of energy is hardly possible with high efficiency, due to change of the fields along all single accelerating structures while regulating power or beam current.
An original and efficient power regulation scheme is proposed in patent SU 1119599, where a several section TW LINAC is powered in a reversed sequence, increasing effective utilization of RF power along the LINAC. A disadvantage of this approach is that a TW section is not efficient at low fields, and at low electron beam velocities in the buncher section.
In U.S. Pat. No. 2,920,228 a LINAC with two TW sections, and a parallel RF feed is described. This arrangement has a disadvantage in that the acceleration efficiency is low due to having a TW accelerating structure, and due to the high complexity of the arrangement.
U.S. Pat. No. 3,070,726 describes a TW LINAC with two TW sections, and a prebuncher. The TW sections are powered in series, and contain a phase shifter and a power adjustment RF circuit. This TW LINAC has a complex circuit, and it does not achieve maximum efficiency of acceleration.
U.S. Pat. No. 4,118,653 proposes a TW buncher section cooperative with an SW accelerating section so as to increase the accelerating gradients in the second section. In this configuration, bunching of the beam is performed in a lower shunt impedance structure, which does not allow the LINAC to operate efficiently, and usually, an external focusing coil is required around the first section to achieve the required performance.
U.S. Pat. No. 4,286,192 describes a method of regulating energy using shorts in the side cavities of the SW structure. Some disadvantages of this method are that the mechanical adjustments are done in the accelerator vacuum envelope, and the energy range is narrow.
Chinese Patent No. CN202019491U discloses a side-coupled SW accelerator that adjusts the electron beam energy by adjusting the accelerating gradient of each of two segments of accelerating tubes. However, this approach too has disadvantages in that the accelerator has a large width, the microware feeding system is complex, and it cannot provide electron beams of low energy (≦1 MeV).
US 20140185775 A1 patent describes a two section standing wave electron LINAC with continuously adjustable energy for medical imaging and other medical applications, as an alternative to X-ray tubes. The disadvantages of this arrangement include that two SW sections are used with a parallel input circuit, and the LINAC is very sensitive to beam loading, so both sections need to be tuned with high precision to match the resonant frequencies for a maximum energy gain operation, and the LINAC may not produce maximum possible dose rate in broad range of its parameters.
U.S. Pat. No. 8,942,351 B2 patent describes a TW LINAC with electron beam energy regulation using switching electron beam current, and therefore setting different beam current loading points in the LINAC. This approach has several disadvantages, the first being similar to the one illustrated in FIG. 1, where optimization of beam dynamics and capture in the TW LINAC is difficult to do at various field distributions due to a beam loading effect. Also, an external solenoid is required for beam focusing, and the remaining RF power after accelerating is complete is lost in an RF load, which has to be used to ensure a TW operation regime.